Enriching nanoparticles in an aqueous solution is commonly practiced for various applications. Despite recent advances in microfluidic technologies, a general method to concentrate nanoparticles in a microfluidic channel in a label free and continuous flow fashion is not yet available, due to strong Brownian motion on the nanoscale. Recent research of thermophoresis indicates that thermophoretic force can overcome the Brownian force to direct nanoparticle movement. Coupling thermophoresis with natural convection on the microscale has been shown to induce significant enrichment of biomolecules in a thermal diffusion column. However, the column operates in a batch process, and the concentrated samples are inconvenient to retrieve. The present inventors have recently designed a microfluidic device that combines a helical fluid motion and simple one-dimensional temperature gradient to achieve effective nanoparticle focusing in a continuous flow. The helical convection is introduced by microgrooves patterned on the channel floor, which directly controls the focusing speed and power. Here, COMSOL (software, Burlington, Mass.) simulations are conducted to study how the device geometry and flow rate influence transport and subsequent nanoparticle focusing, with a constant temperature gradient. The results demonstrate a complex dependence of nanoparticle accumulation on the microgroove tilting angle, depth, and spacing, as well as channel width and flow rate. Further dimensional analyses reveal that the ratio between particle velocities induced by thermophoretic and fluid inertial forces governs the particle concentration factor, with a maximum concentration at a ratio of approximately one. This simple relationship provides fundamental insights about nanoparticle transport in coupled flow and thermal fields. The study also offers a useful guideline to the design and operation of nanoparticle concentrators based on combining engineered helical fluid motion subject to phoretic fields. Using the optimal device geometry and operation condition, nanoparticle enrichment in a simple flow-through process is further demonstrated in a microfluidic channel.
More specifically, enriching nanoparticles in an aqueous solution is a critical step for the preparation of nanomaterials and detection of nano-analytes. While conventional methods such as high-speed centrifugation and ultrafiltration are effective, these batch processes require bulky instruments, extensive infrastructure and long processing time. Yields of such devices are often variable due to aggregation associated with pelleting or caking. Recent advances in lab-on-a-chip technologies have led to the development of portable microfluidic devices for microparticle separation and concentration, employing various physical mechanisms such as centrifugation, inertia-driven flow, dielectrophoresis, optical trapping and acoustophoresis. However, these approaches are mostly ineffective for nanoparticle processing due to scaling of the involved forces with the particle volume and strong Brownian motion of submicron and nanometer-sized species. Microfluidic filtration devices based on the size exclusion principle face similar challenges to their macroscopic counterparts, such as clogging and high back pressure. Capillary electrophoresis, including recent development utilizing the ion concentration polarization phenomenon, is popular for biomolecule purification and enrichment. Unfortunately, the high electric field is detrimental to vesicles and organisms. Affinity capture on solid beds or by magnetic particles 1 have been successfully demonstrated to separate nanovesicles and viruses, but the requirement of affinity interaction limits their application to species with unknown surface biochemistries. A general method to focus nanoparticles physically in a microfluidic channel in a label free and continuous flow fashion is not yet available.
Recently, thermophoresis has been proposed as a physical strategy to control the migration of molecules and colloid particles in aqueous solutions. Thermophoresis refers to the migration of solute species under a temperature gradient. It has long been studied as a means for atomic and molecular enrichment. Initially, thermophoresis was applied to species in the gas phase, and shown to concentrate isotopes when it is coupled with natural convection in a thermal diffusion column. Only in the past few decade has thermophoresis been investigated carefully in the aqueous phase, and newly demonstrated applications include colloidal crystallization, droplet and bubble manipulation as well as molecular assembly. Employing a thermal diffusion column, researchers predicted concentration enhancement of macromolecules by order of magnitude in the liquid phase, and empirically observed enrichment of DNA, organic compounds and polymers.
A thermal diffusion column, however, operates in a batch process. It is a closed, vertical capillary tube with a large aspect ratio, and a temperature gradient is generated across the capillary width. A thermophobic species in the column migrates along the temperature gradient to the cold wall. At the same time, thermal expansion of the solvent introduces natural convection that sweeps solute along the cold wall, and recirculation of the solute is inhibited by the thermophoretic force. As a result, the solute accumulates at a corner where drag forces are low. The steady state could be reached as drag, Brownian and thermophoretic forces are balanced. The local concentration has been found to increase by orders of magnitude compared to the bulk. The steady-state concentration factor scales exponentially with the temperature difference and aspect ratio. However, since the temperature gradient controls both the thermophoretic motion and natural convection, it is difficult to study the interplay of the two transport mechanisms and optimize the enrichment, as well as to extract the concentrated species from the capillary.
As mentioned above, conventional methods use temperature gradients to increase the concentration of macromolecules. However, their throughput is often low because they are batch processes. Two conventional processes—convection and thermophoresis—are driven by temperature gradients; but they make it difficult to separately control the two transport processes, which also limits the throughput. In addition, the temperature gradient is limited to small volumes heated by a laser, which is the enrichment location, and it is difficult to retrieve the concentrated species.
Others have used both electrodes and fluids to generate a temperature gradient. Without engineered convection, the concentration effect happens in one dimension and the enrichment factor is very low (on the order of 1).
Still others have described coupling temperature gradient with simple laminar flow to separate nanoparticles. These methods are only applicable to a plug flow, which results in a batch process with very limited sample volume. These approaches are useful for analysis, but not sample preprocessing, and are not practical for nanoparticle processing.
The present inventors have discovered a continuous approach to sample processing capable of higher throughput, focusing in two orthogonal directions (thus yielding high enrichment factors on the order of, for example, 10-100), separate and independent control of thermophoresis and convection (thus allowing optimization of both throughput and/or enrichment) and gentle operation conditions for biological species. This combination of features has not been delivered by methods heretofore described in the prior art.
The advances of nanotechnology demands new approaches to process nanoparticle samples in a easily-operable, scalable and portable fashion. Conventional methods, such as ultracentrifugation and ultrafiltration require bulky devices and/or extensive manual operations. Microfluidic technologies, albeit effective for processing micro-particles, are limited for continuous, label-free enrichment of polymeric nanoparticles and biological nanovesicles, due to strong Brownian motion of nanospecies in solutions. Recent research of thermophoresis in aqueous solutions indicates that thermophoretic force can overcome Brownian force to direct nanoparticle movement in a biocompatible environment. Coupling thermophoresis with natural convection on the microscale has been shown to induce significant enrichment of biomolecules in a closed capillary called a thermal diffusion column, yet the column has many practical limitations for separation applications. Embodiments of the present invention present a microfluidic device that can concentrate submicron particles in a simple flow-through process. In an embodiment, the device couples an engineered helical flow and a mild, one-dimensional temperature gradient to achieve nanoparticle focusing. Here, the device geometry, flow rate and temperature gradient experimentally are optimized to enhance the focusing effect. As thermophoresis is ubiquitous in fluids, the approach applies universally to suspended soft nanoparticles without the need of labeling and allows continuous retrieval of concentrated species in a simple flow-through process. This microfluidic solution holds a promise to nanoparticle processing in resource-limited settings.
As such, there remains a need for apparatuses and methods of manufacturing and using the apparatuses which provide nanoparticle focusing by coupling thermophoresis and engineered vortex in a microfluidic channel which are able to overcome the above disadvantages. Embodiments of the present invention are designed to meet these needs.